1 / 26

Embedded Control Applications II MP10- 1

Embedded Control Applications II MP10- 1. Embedded Control Applications II MP10- 2. Control of a DC servo-motor.

baby
Download Presentation

Embedded Control Applications II MP10- 1

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Embedded Control Applications II MP10-1

  2. Embedded Control Applications II MP10-2 Control of a DC servo-motor - The control of a DC servo motor is another common task in robotics / mechatronics; DC servo-motors combine regular DC-motors with a gear-box and an encoder/potentiometer to form a position control loop • Being position controlled, the drive shaft of a servo-motor can only assume a limited range of angular positions (typically ±90° or less) • The set-point is a Pulse Width Modulated (PWM) signal with a period of commonly around 20 ms and duty cycles of 2% to 10% (~0.5 ms to ~2.5 ms)

  3. Embedded Control Applications II MP10-3 Control of a DC servo-motor Encoder / Potentiometer Power amplifier Gear-box DC motor

  4. Embedded Control Applications II MP10-4 Control of a DC servo-motor - Servo-motors have 3 wires: Vsupp,VGNDand signal - Only the signalline interfaces to the microcontroller – the current carrying supply lines need to be connected to a sufficiently powerful supply; typical supply voltages range between 6 V and 30 V red 6 V DC 10% duty cycle black yellow 20 ms

  5. S Embedded Control Applications II MP10-5 Control of a DC servo-motor A small DC servo-motor is to be driven using an edge aligned PWM signal on P7.3 (period: 20 ms). The duty cycle is to be controlled by the analogue voltage applied to ADC channel 4, which is also logged on a terminal connected to ASC port S0 (57600 bps) P5.4 5V PORT 5 PORT 7 PORT 3 10 V 0V RxD P7.3 TxD 0V

  6. Embedded Control Applications II MP10-6 Control of a DC servo-motor - This is essentially the program developed in lecture MP9; the period of the PWM signal has to be adjusted to 20 ms and the duty cycle needs limited to the range from 3 % (0.6 ms) to 10 % (2 ms) - To produce the required 20 ms signal (50 Hz) the PWM module needs to be set up with a pre-scale factor of 1/64; a 12-bit resolution is to be expected (mode: 0), i. e. 2010-3/4096  4.8810-6 s  5µs

  7. Embedded Control Applications II MP10-7 Control of a DC servo-motor - Running the modified program on the C167 confirms a 20 ms period with pulses ranging from 0.6 ms (setting: 0 %) to 2 ms (setting: 100 %)

  8. Embedded Control Applications II MP10-8 Control of a DC servo-motor - The terminal logs the current position as a percentage of the range of admissible pulse widths

  9. Embedded Control Applications II MP10-9 Control of a stepper motor [1] - A stepper motor is an electromechanical device which converts electrical pulsesinto discrete mechanical movements - The shaft or spindle of a stepper motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence - This sequence is directly related to the direction of rotation of the motor shaft; the speed of the rotation is directly related to the frequency of the applied pulse sequence

  10. Embedded Control Applications II MP10-10 Control of a stepper motor Stepper motors have the following characteristics: - The rotation angle of the motor is predictably related to the input pulse pattern - The motor has full torque at stand-still (if the windings are energized) - Precise positioning and repeatability of movement; good stepper motors have an accuracy of 3 – 5 % of a step – this error is non-cumulative from step to step - Excellent response to starting, stopping, reversing

  11. Embedded Control Applications II MP10-11 Control of a stepper motor Stepper motors have the following characteristics: - Stepper motors are brushless and thus very reliable; their life span usually only depends on their bearings - They allow for accurate open-loop control; the position can be tracked simply by counting pulses - They allow for very low speed synchronous operation with loads that are directly coupled to the shaft - Improper control may cause resonance phenomena - Difficult to operate at extremely high speeds

  12. Embedded Control Applications II MP10-12 Control of a stepper motor Three different kinds of stepper motors exist: - Variable-reluctance (VR) stepper motors consist of a soft iron multi-toothed rotor and a wound stator - Energizing the stator windings with DC currents causes the poles to be magnetized - Rotation occurs when the rotor teeth are attracted to the energized stator poles

  13. Embedded Control Applications II MP10-13 Control of a stepper motor Three different kinds of stepper motors exist: - Permanent-Magnet (PM) stepper motors (‘tin can’) are low cost and low resolution type motors – typical step angles range from 7.5° to 15° - The rotor no longer has teeth (cf. VR motor), but is magnetized with alternating north and south poles - The increased magnetic flux intensity gives the PM motor an improved torque characteristic

  14. Embedded Control Applications II MP10-14 Control of a stepper motor Three different kinds of stepper motors exist: - Hybrid (HB) stepper motors combine the best features of PM and VR type stepper motors; step angles vary from 3.6° to 0.9° (100 – 400 steps per revolution) - The rotor is teethed with an axially magnetized concentric magnet around the shaft - The teeth on the rotor help guiding the magnetic flux; this leads to increased performance

  15. Embedded Control Applications II MP10-15 Control of a stepper motor - The stator windings need to be energized in such a way as to generate a rotating magnetic field; the rotor follows this field due to magnetic attraction - Two-phase example:Energizing the windings using a B-A-B-A-B-… pattern leads to clockwise rotation - The rotational speed depends on the frequency of the alternating sequence

  16. Embedded Control Applications II MP10-16 Control of a stepper motor - The torque of a stepper motor depends on the step rate as well as the intensity of the magnetic flux in the windings which, in turn, is proportional to the drive current - A stepper motor usually has 2 phases; more complicated designs with 3 and even 5 phases exist - A pole can be defined as one of the regions where the magnetic flux density is concentrated; there are poles on both the rotor as well as on the stator - Increasing the number of poles on rotor and/or stator leads to smaller basic stepping angles (full step)

  17. Embedded Control Applications II MP10-17 Control of a stepper motor - Example: Unipolar 2-phase stepper motor with one pair of poles per phase and one pair of rotor poles - The flux can be reversed by switching the supply from phase A/B to phase A/B

  18. Embedded Control Applications II MP10-18 Control of a stepper motor - Example: Bipolar 2-phase stepper motor with one pair of poles per phase and one pair of rotor poles - The flux can be reversed by swapping the + and - terminals of the supply - 8 full step positions are possible (basic step angle: 45°)

  19. Embedded Control Applications II MP10-19 Control of a stepper motor - The most common stepping modes are wave drive, full step driveand half step drive - In a wave drive system only one phase is energized at any given time; sequence: A  B  A  B … leads to steps from 8  2  4  6 (see MP10-18) - In a full step drive system two phases are energized at any time; sequence: AB  AB  AB  AB … leads to steps from 1  3  5  7 (see MP10-18) - A half step drive system combines the above two modes; sequence: AB  B  AB  A  AB  B  AB  A … (1  2  3  4  5  6  7  8)

  20. Embedded Control Applications II MP10-20 Control of a stepper motor - The advantage of full step driveover wave drive is that, at any given time, a full step system uses 50% of the available windings whereas the equivalent wave drive system only uses 25% - Furthermore, unipolar stepper motors only use 50% of each winding to build up the magnetic flux; bi-polar stepper motors on the other hand use the full winding and therefore produce more torque - Microstepping systems continuously vary the current amplitude in the windings to break up a basic step into many smaller discrete steps

  21. Embedded Control Applications II MP10-21 Control of a stepper motor - The stiffness of a stepper motor can be increased by increasing its holding torque (TH); moving the drive shaft away from an equilibrium position (rotor and stator poles are aligned) leads to an opposing torque which increases until TH is reached - Beyond the holding torque, the rotor position becomes unstable and it moves until it is aligned with the next stator pole

  22. Embedded Control Applications II MP10-22 Control of a stepper motor - The torque vs. speed characteristic of a stepper motor indicates its pull-in curve (defines a region at which the motor can be started/stopped without loss of synchronism)… - … as well as its pull-out curve (limits the slew region, i. e. the region within which the motor can be operated without loss of synchronism)

  23. Embedded Control Applications II MP10-23 Control of a stepper motor - The time-domain response of a single step is subject to load conditions and the maximum required acceleration - Driving the motor at frequencies near the natural frequency of the rotor can lead to resonance; this resonance manifests itself in a sudden loss or drop in torque at certain speeds which can lead to loss of synchronism

  24. Embedded Control Applications II MP10-24 Control of a stepper motor - The driver of a stepper motor can be implemented using a microcontroller; the controller needs to produce the required pulse sequence and interface to an array of inverters or power MOSFETs - This would only be done for educational purposes; in the ‘real world’ a stepper motor driver chip would be used (cost: ‘a few dollars’) - This reduces the task to the provision of a pulse sequence, the frequency of which defines the rotational speed, and a directional signal (fw. / rev.)

  25. Embedded Control Applications II MP10-25 Control of a stepper motor - A typical design of a stepper motor driver is shown below; note that the transistors of the power amplifier often have to be implemented externally

  26. Embedded Control Applications II MP10-26 Further reading: [1]Douglas W. Jones, Control of Stepping Motors – A Tutorial,http://www.cs.uiowa.edu/~jones/step/, accessed: January 2005 [2]ELF/DWARF, Free Standards Group – Reference Specifications, www.linuxbase.org/spec/refspecs/, accessed: January 2005 [3]The GCC Project, Free Software Foundation,gcc.gnu.org/, accessed: January 2005

More Related